"It's not been great, actually," Musk told the Times. "I've had friends come by who are really concerned."

So how much sleep do we really need?

Like most health factors, there isn't a one-size- fits-all answer — sleep needs vary from person to person. There are some incredibly rare people who can actually get by on a few hours of sleep per night, and others on the opposite end of the spectrum that doctors refer to as a " long sleepers" because they need 11 hours nightly.

But research on sleep can help you figure out how much you need and how to better get a night's rest. Here are five facts that will help you figure out what your personal sleep patterns are and how they compare to the rest of the population.

According to CDC data, getting less than seven hours per night is "associated with increased risk for obesity, diabetes, high blood pressure, coronary heart disease, stroke, frequent mental distress, and all-cause mortality."

Sleep deprivation can also hurt cognitive performance(as many of us have probably experienced), and that in turn "can increase the likelihood of motor vehicle and other transportation accidents, industrial accidents, medical errors, and loss of work productivity," the CDC says.

You have a natural chronotype, or body clock, that determines when you are most comfortable sleeping and being awake

"Where you define owl or lark is really arbitrary," says Dr. David Welsh, an associate professor studying circadian clocks at UC San Diego.

Welsh says that if you look at large surveys of populations, you get a normal distribution of chronotypes — most people have fairly "average" chronotypes, some prefer to get up a bit earlier or later, and small groups naturally rise extremely early or late. There's no line that distinguishes different chronotypes.

But we all have an internal schedule that makes us feel awake or sleepier at different times of day. Because of factors including hormone levels, genetics, and light exposure, some of us are more alert in the mornings and some of us prefer times later in the day.

If your schedule isn't aligned with your chronotype, you will feel tired and out of sync.

The amount of sleep you need changes throughout your life

National Sleep Foundation

The seven-to-nine-hour rule is standard for adults, but kids need much more sleep, while some older people need less.

This chart by the National Sleep Foundation shows how these requirements change as kids grow up.

In addition to sleep hour needs changing, chronotypes change throughout life as well.

According to Roenneberg's book, young children naturally tend to be more morning oriented. Around puberty, they're more likely to shift into a night owl chronotype, which tends to shift back to an earlier chronotype after age 20.

There are things you can do to adjust your natural chronotype

While your sleep needs (when you feel alert and how much sleep your body requires) are mostly genetic, there are certain things you can do to adjust your schedule and make it a bit easier to get up or go to sleep earlier.

Our bodies respond to light, especially the powerful natural light of the sun. Being exposed to that light in the morning tells our body that it's time to be alert and moving. At night, sitting in the dark stimulates the production of the hormone melatonin, which helps us relax and fall asleep (we mess with this process by looking at bright light from smartphones).

But we can adjust this to a degree by controlling our exposure to light. This process, called entrainment, is what our bodies have to do when we go to a different time zone — this is why we get jet lagged. But we can also use this to train our bodies to get up and go to sleep earlier by exposing ourselves to natural light in the morning and avoiding bright light at night.

This won't turn you into a morning person, but it can make prying the covers loose just a little less painful.

Your sleep needs are personal; try to figure out what works for you

Sometimes new research will come out, and people will claim something like "studies have found that seven hours is the optimal amount of sleep — not eight."

But as interesting as any sleep research is, different people simply have different needs. The findings of one study don't translate into recommendations for everyone. In the case of sleep, experts recommend figuring out what works best for you.

If you can let yourself sleep naturally for a few days to a week — going to bed when you are tired and waking up whenever is natural, preferably while limiting alcohol and caffeine — you'll have a better idea of your individual needs. During those days, try to get some sun and some exercise.

If you do all that and still have trouble sleeping, it might be time to talk to a doctor. You could be one of the large percentage of the population with undiagnosed sleep apnea, especially if you snore. Or you could have some other disorder that can be addressed.

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